Seasonal trivalent influenza virus vaccine (TIV) is recommended for all pregnant women because they are considered at high risk for complications, hospitalization, and death from influenza. Vaccination in pregnancy can also provide protection for infants from 0-6 months, a high risk group for whom vaccination is not approved. However, there are virtually no data on predictors of adequate maternal antibody response or sufficient antibody transfer to the newborn. Studies of nonpregnant adults and animals as well as our own preliminary data on antibody responses following TIV in pregnant women demonstrate that psychosocial stress and obesity are two key risk factors that warrant attention in this regard. Study Design: We will examine effects of psychosocial stress and obesity on 1) antibody responses to TIV in pregnant women and 2) antibody transfer from the mother to the newborn. This study will include 180 women (90 obese, 90 non-obese) who will be vaccinated during the 2nd trimester of pregnancy (13-28 weeks gestation). Maternal antibody levels will be assessed prior to vaccination, at one month post-vaccination and at the time of delivery. Newborn antibody levels will be measured via cord blood at the time of delivery. Data will be collected across three influenza seasons. In each season, 60 women will be assessed; 30 obese and 30 non-obese who will be similar in age, race, parity, and socioeconomic status. Anti-influenza antibody titers to A/H1N1, A/H3N2, and B strains in each year will be determined by the hemagglutination inhibition (HAI) test. Hypothesis 1: Among pregnant women, both stress and obesity will predict decreased likelihood of achieving a protective antibody response at one month after TIV. Effects of obesity will be the most pronounced among highly stressed women. Hypothesis 2: Greater maternal stress and obesity will predict decreased likelihood of protective antibody levels in newborn cord blood, with the most robust effects among those with both risk factors. We will examine four pathways which may contribute to effects in newborns: 1) impaired magnitude of maternal antibody response (Hypothesis 1), 2) poorer maintenance of maternal antibody levels from one month post-vaccination to delivery, 3) less efficient maternal-fetal transmission (newborn cord blood/maternal antibody ratio), and 4) higher rates of preterm birth. This study tests the innovative hypotheses that psychosocial stress and obesity substantially impair maternal antibody responses and antibody levels in newborns following maternal influenza vaccination in pregnancy. If our hypotheses are confirmed, we will identify and quantify the impact of novel risk factors for poor immune protection in two populations vulnerable to influenza complications, pregnant women and infants. High dose vaccine or two dose schedules are now available for other high risk groups who show poor responses to traditional TIV including older adults, children, and transplant recipients. This study will determine if it is justified to re-evaluate influenza vaccine recommendations for pregnant women with specific risk factors.